1. Field of the Invention
The invention relates to a phase-locked loop (PLL), and in particular to a phase-locked loop utilizing multi-band voltage control oscillator (VCO).
2. Description of the Related Art
A VCO requires capability to tune or adjust frequency over a range sufficient to cover a specified range of operating frequencies under expected worst-case conditions. A common use of a VCO is within a phase-locked loop (PLL). Depending on the application for which the PLL will be used, the specified operating frequency range may be a single frequency or a range of frequencies. The worst-case conditions may result from different supply voltages, variations in manufacturing process, component tolerances, and so on.
Conventionally, most PLLs employ a single-band VCO that provides a single range of frequencies (or operating band). For a conventional PLL with a single-band VCO, the tuning range of the VCO needs to cover the specified operating frequency range under the expected worst-case conditions. The gain of the VCO may thus be relatively large in order to obtain a large tuning range. However, a large VCO gain is typically undesirable for sensitivity to noise and degraded jitter performance of PLL.
A multi-band VCO provides multiple ranges of frequencies (or multiple operating bands) that collectively represent the overall tuning range of the VCO. By dividing the tuning range into multiple overlapping operating bands, the VCO gain may be reduced since the full range of the control signal corresponds to a smaller range of frequencies for one operating band. The smaller VCO gain of the multi-band VCO may then ameliorate many of the disadvantages of the single-band VCO.
Because a multi-band VCO can operate in any of the multiple operating bands, the proper operating band needs to be selected for use at any given moment. Thus, a PLL utilizing a multi-band VCO and having an automatic switching mechanism to properly switch the operating bands of the multi-band VCO is desirable.